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A multi-reference configuration interaction (CI) method based on multiple Hartree–Fock (HF) solutions is applied to study the permanent dipole moment of the LiH, BH, FH, and H2O systems with the minimal atomic basis. The LiH was also calculated with the double-zeta atomic basis. This method uses several HF solutions as references to expand the state functions. For each one of the systems studied, several HF solutions were obtained in the appropriate point and spin symmetry classes, i.e., 1Σ+ and 1A1. With each of these solutions is generated a distinct basis of the same full CI space. The set of these bases is a system of generators of the full CI space and the multi-reference HF (MRHF) bases are built mixing configuration state functions (CSFs) originated from distinct HF solutions. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

The multiple-scattering Xalpha molecular cluster model is used to carry out self-consistent calculations of the electronic states of an isolated P vacancy in GaP. A fully occupied singly degenerate a1 level and a threefold-degenerate t2 level occupied by one electron are found within the band gap. This configuration indicates that the centre is unstable with respect to Jahn-Teller distortions. The vacancy states a1 and t2 are quite extended showing an appreciable charge distribution in the next-nearest neighbours of the vacancy (P atoms). The role played by these results in the explanation of the properties of the GaP:O centre is discussed.

The multiple-scattering Xα method within the framework of the Watson-sphere-terminated cluster model was used to carry out electronic structure calculations of a phosphorus vacancy and an oxygen substitutional impurity replacing phosphorus in GaP. The calculations were performed by considering different electronic configurations of the oxygen 2p atomic states, and the results were compared with those derived from the two theoretical models recently proposed to describe the experimental data on the GaP:Op system. An alternative description is suggested which may explain the well known large relaxation that occurs after the capture of a second electron by the center.